The present invention relates to multifunction fiber optic couplers.
Single-mode fiber optic couplers are useful as signal combiners and distributors in multiple terminal systems such as those for bringing wideband services to consumers. Some of these systems need high resolution wavelength division multiplexer (WDM) couplers. A multicoupler system that is used in fiber amplifiers taps off a small percentage of the input signal for monitoring purposes and couples the remainder of the signal along with the pump power to the amplifying fiber. There are many other uses for multicoupler systems.
Fiber optic couplers can be made by drawing and fusing two or more single-mode optical fibers to form a biconical taper structure. The application of heat and tensile load causes the necking down of each fiber into a biconically-tapered section which fuses together with similar sections of adjacent fibers since the fibers are softened.
Various coupler properties can be improved by inserting the fibers into a glass tube prior to heating and stretching the fibers, thereby resulting in the formation of an "overclad coupler". The process involves inserting the fibers into a glass tube, evacuating the tube, and heating a region of the tube to cause it to collapse onto the fibers. The refractive index of the glass tube is lower than or equal to that of the claddings of the fibers. The central portion of the collapsed region is drawn down to that diameter and coupling length which is necessary to obtain the desired coupling properties.
The aforementioned multifunction and other coupler systems can be formed by connecting together two or more simple couplers by sections of optical fiber. However, such coupler systems are relatively expensive and can have reliability problems. Also, these systems are cumbersome and, because of their interconnecting fibers, occupy an inordinate amount of space. Systems installers would therefore prefer to handle unitary coupler devices which include multiple couplers and which perform multiple functions.
While planar technologies can be employed to produce multi-coupler systems, small port count devices have little margin for loss, and planar manufacturers have difficulty meeting the optical requirements in a commercially viable way. The present invention can be employed to produce these small port count devices very cost effectively as compared to planar fabrication techniques. In accordance with this invention, these disadvantages are overcome by employing unitary fiber-type coupler devices. In addition, monolithic coupler technology can be used to form environmentally stable multifunction coupler systems.
One particular multifunction coupler for which there is a present need functions as a combination signal combiner and signal splitter. For example, a 2.times.4 coupler combines two signals and splits the combined signals equally into four output signals. Attempts have been made to form an achromatic 2.times.4 coupler by the aforementioned overclad technique, whereby optical fibers are inserted into a glass tube that is collapsed onto the fibers and stretched to form a single coupling region of reduced diameter. The resultant devices have not shown the required degree of achromaticity.
The device of the present invention accomplishes the achromatic 2.times.4 coupling function in a unitary coupler device having two concatenated coupling regions. The unique design of this unitary coupler is such that two fibers that couple in the first coupling region do not couple in the second coupling region, thus avoiding that interference effect that would result in a variation in output power as a function of wavelength. This feature of the invention also enables the fabrication of coupler devices that perform other functions.
It is an object of the present invention to provide an improved fiber optic coupler that will overcome the heretofore noted disadvantages of prior art coupler devices and systems. Another object is to provide a unitary fiber-type concatenated coupler device that has two or more tapered coupling regions and that is so constructed that there is no resonance caused by two fibers coupling to each other in two coupling regions. A further object is to provide a monolithic fiber-type device that performs a plurality of functions.
As used herein, the term "active fiber" means an optical fiber having a core and cladding; such a fiber is capable of efficiently propagating light. Active fibers are contrasted with "dummy" fibers that do not contain cores.